Thermodynamics
by fluffernutter
Summary: Energy is neither created nor destroyed. It simply changes form. One-shot.


**Disclaimer**: I own my cat, this computer and the can of Bawls that I'm drinking. Not this show…so the only recompense I'll accept will be in the form of reviews.

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_"Energy may neither be created nor destroyed."_

**The First Law**

_"The increase in the internal energy of a system is equal to the amount of energy added by heating the system, minus the amount lost as a result of the work done by the system on its surroundings."_

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It was irritating—that smug grin reaching his right eye and slightly crinkling the corner of his left. That was two for three sparring matches where she had been bested. He didn't have to taunt her with words, and he knew it. Offer _her_ a hand up after being shamed by his victory? Hah! She didn't need charity from him of all people. She roughly shoved his hand away.

Katara pushed herself into a crouch, feeling her temper rising. _Focus! Calm down! You need to focus._ She shook her head, grating her teeth against the anger that was setting them on edge. "One more time," she said, ineffectively dusting herself off with hands clenched into fists. Strands of hair were plastered with sweat and dirt to her cheeks, her forehead, on the back of her neck. She refused to acknowledge the way sweat glistened off his bare arms and chest.

He let out a sigh. "Look, you obviously aren't focused enough for this right now. Let's wash up and get started on dinner for everyone." He gathered up his tunic and shrugged it back on before turning to walk back to their campsite.

That was it. He was _dismissing_ her? "I don't need you to tell me what I can or can't do!" He didn't stop or turn around, merely giving a hand a brief flutter in her direction as he continued on his way.

She bent the sweat from her body, removing the offending hair from where it clung to her skin and twisted her torso into a bending stance. She vaguely felt a stone cutting into her heel where she dug it back, bracing it into the ground. "Don't tell me you're too scared of losing this time. Coward." He stopped. _Good_, she thought. _Now we're getting somewhere_.

He whirled and started towards her. "What is your _problem_?! What exactly have I done to get you so angry?" He raked a hand through the messy hair falling into his eyes. "You seemed so much calmer lately, but ever since I helped you with dinner last night, you're on the warpath again! Urgh!!" He let out a frustrated sound and smoke plumed from both nostrils.

"You're here. Isn't that enough reason for me to be angry?" She stood up straight, fists closing at her sides as she felt the heat of anger rising in her again.

"Then tell me to go away! Ignore me! Don't say anything to me at all!" He was within reach now. "But don't start treating me like I almost belong and then start blowing up at me again without a reason!"

"I _do_ have a reason! I couldn't...I mean, I didn't..." She broke the glaring match first, eyes darting to either side before resting on his feet. Why _was_ she so angry? Until the night before, they had seemed to forge an understanding that nearly approached camaraderie. She thought about that night.

And whether it was him with whom she was so furious...or herself.

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**The Second Law**

_"The entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium."_

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"...and so if you have a lot of glasses at your house, you shouldn't be earthbending." He listened to her snort out some laughter as they chopped up the ingredients for dinner. _Not getting Uncle's sayings right isn't _so _bad_, he thought with an inward grin.

"No wonder they didn't feel threatened when you joined. How intimidating is someone that can't even get a proverb straight?" He chucked a small piece of carrot at her head. "Hey!" She playfully hit him in the ear with a small splash of water. "Why don't you start the cooking fire, Zuko? That way we might _actually_ be able to finish on time." He smiled as he ignited the firewood and spread out the flames to heat the pot evenly. He glanced back and caught her staring at him, with a look he wasn't quite sure he felt comfortable placing.

Katara watched the expression on his face and felt a similar one grace her own. Was this really the same boy that always grimaced? That traded his anger for melancholy and still wouldn't smile? Light from the flame flickered in the gold from his eyes, and she tried to ignore the mild heat rising in her cheeks. As he glanced back, she noticed that she had been staring, and her face flushed. "I-I-uh," She stammered briefly before feeling a sharp pain in her hand.

"Ow!" Zuko watched her drop the blade and jerk her hand away from the table. He saw a scattering of crimson on the surface and came over in a few short strides. "Let me see." He grabbed the wrist of her injured hand and turned it to see the backs of her knuckles. A deep, clean gash cut across two of her fingers, bleeding at a slow and constant rate.

She was interrupted from grabbing her water pouch by the feel of his hand (_so warm..._) against hers. Unbidden, her heartbeat sped up slightly, thrumming a low, syncopated rhythm in her temples. She would not have expected his touch to be so gentle.

"You got yourself pretty good, huh?" He looked at her unresponsive face. "Uh, you should be able to take care of this with no problem, right?" She glanced up and met his gaze, wide-eyed. He noticed the coloring in her cheeks and the "kitchen's" temperature appeared to climb a few degrees. He felt a mild discomfort rise in his stomach, seemingly to lodge itself in his throat. He cleared it. "Um, Katara?"

The sound of her name from his voice startled her back into the moment. Her hand in his, the close proximity of their bodies, the concern and uncertainty in that golden gaze. She jerked her hand away, bristling at the traitorous response her body was having. She fumbled her water pouch open with her other hand. "Why don't you back off? I don't need any help."

His eyes widened before she saw his face retreat into a neutral mask of indifference. "Are you sure you're—"

"I said I don't need your help! Why don't you just leave so I can finish?" A scowl had settled over her features, anger over (_warm, firm skin_) his distraction bubbling up.

Zuko walked out, leaving her to her thoughts as she enveloped her hand in water. _What in the world was going on_?

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**The Third Law**

_"As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value."_

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"I mean, it's not like it was _my_ fault you cut yourself!"

She stomped her foot and stared up at him, faces close enough for her to feel the heat emanating from his. "It _was_ your fault!"

He couldn't believe how irrational she was being. "It was an accident, and I wasn't even anywhere near you! How could it have _possibly_ been my fault?"

"Because I was staring at _you_ when I did it!"

His mouth was open to respond, but it just hung there in confusion instead. He watched as she turned away, giving him a brief sidelong glance.

Katara was no longer yelling, her voice subdued and unsure. "I _can't_ be looking at you like that. I _can't_ like you. Not you. Not after...everything."

Zuko's mouth slowly closed, and he realized he had been holding his breath. He let it out, relieved, in a soft chuckle.

She glared. "I don't see what's so funny—"

"That's all it was? You don't hate me?"

Her cheeks tinged with pink, she replied, "Look, I'm not going to..." He tilted her face towards him with (_warm, so warm._) fingers beneath her chin. "...repeat..." She could feel his breath on her lips. "...myself," she whispered. Her mind was a whirlpool rushing about, as a warm undercurrent of emotion rose up. She pushed back against the warmth with a heat of her own, getting lost in the lull of the feeling.

He pulled back afterwards and it was a mild consolation that he looked as breathless as she felt. He gave her that infuriating (_endearing_) smirk.

"So, does that mean I win this round?"

It was still irritating, but she'd let him win. Just this once.

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**A/N**: My first sorry lil' attempt at AtLA fanfiction. One-shot, though I have a few plot bunnies that make me want to try a longer fic with the Thermodynamics theme. Review if you like. Debate pairing choices, content or what have you.


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